In the field of petroleum refining, catalytically cracked gasoline is a stock of high-octane number gasoline containing a certain amount of olefin components. Catalytically cracked gasoline is a gasoline fraction obtained by catalytically cracking a heavy petroleum fraction as a stock oil, such as a vacuum gas oil or an atmospheric residual oil, and recovering and distilling the catalytically cracked products. Catalytically cracked gasoline is a primary blending stock of automotive gasoline.
While some stock oils have a small sulfur content and may be subjected to catalytic cracking without treatment, a stock oil for catalytic cracking generally has a relatively high content of sulfur compounds. When an untreated stock oil having a high sulfur content is subjected to catalytic cracking, the resulting catalytically cracked gasoline also has a high sulfur content. Such a gasoline fraction having a high sulfur content would cause environmental pollution if used as a blending stock for automotive gasoline.
Consequently, the stock oil is usually subjected to a desulfurization process prior to catalytic cracking.
A hydrodesulfurization process has hitherto been carried out to achieve the above-mentioned desulfurization in the field of petroleum refining. A hydrodesulfurization process comprises contacting a stock oil that is to be desulfurized with an appropriate catalyst for hydrodesulfurization in a pressurized hydrogen atmosphere at a high temperature.
Catalysts used for hydrodesulfurizing a stock oil for catalytic cracking (e.g., a vacuum gas oil or an atmospheric residual oil) comprise a group VI element (e.g., chromium, molybdenum and tungsten) and a group VIII element (e.g., cobalt and nickel) supported on an appropriate carrier (e.g., alumina). The hydrodesulfurization process is usually conducted at a temperature of about 300 to 400.degree. C., a hydrogen partial pressure of about 30 to 200 kg/cm.sup.2, and a liquid hourly space velocity (hereinafter abbreviated as LHSV) of about 0.1 to 10 1/hr.
In the case of hydrodesulfurizing a heavy petroleum fraction, such as a vacuum gas oil or an atmospheric residual oil, which is a stock oil for catalytic cracking, the processing is carried out at a high temperature and high pressure as stated above. Therefore, strict conditions are imposed on apparatus design, thereby incurring high construction costs. Also, in some cases an undesulfurized stock oil is subjected to catalytic cracking as described above. Even in cases where a stock oil is desulfurized prior to catalytic cracking, there has been a tendency to enhance the catalytic cracking apparatus without adequately desulfurizing the stock oil.
Catalytically cracked gasoline obtained from a desulfurized stock oil contains sulfur in an amount of 30 to 300 ppm by weight (in the whole fraction) and that obtained from an undesulfurized stock oil contains as much as 50 to several thousand ppm sulfur by weight (in the whole fraction). Under these circumstances, there is increasing difficulty in complying with present day environmental regulations.
Catalytically cracked gasoline can be directly subjected to hydrodesulfurization. In this case, however, the olefin components present in the cracked gasoline fraction are hydrogenated to reduce the olefin content, and the resulting cracked gasoline fraction has a reduced octane number. The reduction in octane number is significant when a high rate of desulfurization is required.
Sulfur compounds contained in catalytically cracked gasoline include thiophenes, thiacyclopalkanes, thiols and sulfides. The proportion of thiophenes is large, while the proportions of thiols and sulfides are small.
Sulfur is removed as hydrogen sulfide by desulfurization, but hydrogen sulfide in the gaseous phase reacts with olefins in the catalytically cracked gasoline to produce thiols. In order to attain a certain minimum rate of desulfurization, olefins should be hydrogenated to prevent the production of thiols. Thus, a high desulfurization rate cannot be obtained without being accompanied with a further reduction in octane number.
If catalytically cracked gasoline is desulfurized while its olefin components remain non-hydrogenated, thiols are unavoidably produced. Because thiols are corrosive, they must be made non-corrosive. This is done by converting the thiols to disulfides by a catalytic reaction, which necessitates installation of a sweetening apparatus.
Catalysts used for hydrodesulfurization of catalytically cracked gasoline containing sulfur compounds and olefin components comprise a group VIII element (e.g., cobalt and nickel) and a group VI element (e.g., chromium, molybdenum and tungsten) supported on an appropriate carrier (e.g., alumina) similar to other desulfurization catalysts. These catalysts are activated by preliminarily sulfiding in the same manner as used for treating desulfurization catalysts for naphtha. That is, naphtha is mixed with a sulfur compound, such as dimethyl disulfide, and the mixture is heated to 150 to 350.degree. C. together with hydrogen and passed through a reaction tower packed with the catalyst. The sulfur compound, e.g., dimethyl disulfide, is converted to hydrogen sulfide by reacting with hydrogen at the surface of the active metal of the catalyst. The hydrogen sulfide is further reacted with the active metal to form a metal sulfide active in the desulfurization reaction.
Thus, a reduction in octane number due to hydrogenation of olefins has been a great problem in hydrodesulfurization of catalytically cracked gasoline. There has been a need to develop a technique of efficiently hydrodesulfurizing catalytically cracked gasoline while minimizing the reduction of olefin components.
To meet this demand, the reaction between hydrogen sulfide resulting from desulfurization and olefins must be controlled to thereby control the formation of thiols. However, an increase in desulfurization rate leads to an increase in hydrogen sulfide concentration in the gas phase, resulting in acceleration of thiol formation. In other words, it has conventionally been difficult to achieve a high desulfurization rate while suppressing the hydrogenation reaction of olefins. Rather, it has been necessary to hydrogenate olefins in order to prevent the same from producing thiols to thereby increase the desulfurization rate, which in turn results in a reduction in octane number.